We believe that cofiring appropriate biomass residues with coal represents the lowest cost, lowest risk, shortest term option for effectively reducing greenhouse gas emissions associated with power production, with the possible exception of energy conservation. The four phrases of this statement that require some detailed discussion are appropriate biomass residues, lowest cost, lowest risk, and shortest term.
The phrase "appropriate biomass residues" represents any residual material from any process with renewable and sustainable feed streams. A few common and important examples are most forest products industries (most developed countries plant more trees than they harvest each year) and agriculture (plants are regrown each year at essentially the same rate they are harvested). Some important and common examples of inappropriate biomass are forest products derived from non-sustainable harvesting (for example, destruction of rainforest for use as agricultural or housing land) and some waste materials that are essentially non-renewable (tires and plastics are primarily derived from fossil resources that are essentially non-renewable). Energy crops are a separate consideration that are considered below. Note that these resources are closed-loop on a global scale, even though they may pass through several intermediate stages of use prior to becoming fuel.
The phrase "lowest cost" reflects the combined cost of capital and operations on biomass compared to other renewable energy options. With some exceptions, biomass-derived energy is generally more expensive than comparable fossil energy in an actual, open and fair market, that is, when tax incentives, subsidies, externalities, societal impacts etc. are not considered. However, when biomass is cofired with coal in existing facilities, the incremental cost of the biomass-derived energy is small. While generally more expensive than fossil fuels, biomass cofiring can provide energy that is only marginally more expensive than coal and significantly less expensive than other renewable energies. When the externalities are considered, a strong argument can be made that biomass is cheaper for society than is coal. Specific estimated capital and operating costs for biomass cofiring are $0- $250/kW of biomass capacity and $-0.001 to $0.05 per kWh generation costs. These capital costs are very low compared to virtually any realistic power system today and the generation costs are considerably lower than other forms of renewable energy.
The phrase "lowest risk" reflects the technical risk of biomass cofiring compared to other advanced technologies for CO2 reduction through increased efficiency or carbon sequestration. Most advanced technologies would also benefit from cofiring, as for example a cofired gasifier in an integrated gasification combined cycle system. Prudent application of such technologies is an important energy policy objective, but these systems carry significantly more technical risk and would be believed to have significantly less availability during development than does biomass cofiring. In these senses, biomass cofiring is very low risk.
The phrase "shortest term" indicates that the hardware and fuel handling systems for biomass-coal cofiring can be implemented within a few months at nearly any power station. By contrast, substantial research, development, and demonstration are required for almost all advanced technology and carbon sequestration systems. Biomass-coal cofiring is one of very few technologies that can make almost immediate contributions to net CO2 reductions.
This figure shows how fossil fuel displacement by CO2 neutral fuels (abscissa) compares with increases in efficiency (left ordinate) in reducing net CO2 emissions. For example, the best future technologies (IGCC, for example) may provide as much as a 50% increase in efficiency compared to existing power plants. The same effect on net CO2 emissions can be achieved by cofiring about 33% of a CO2 neutral fuel such as appropriate biomass. The IGCC systems will not be commercially feasible for many years, whereas biomass cofiring can be implemented almost immediately. The right ordinate represents actual efficiencies (based on the higher heating value) of modern, installed, coal-fired systems.
This diagram is the corner of the overall efficiency diagram that is most relevant to current or proven technologies. Here we see that existing facilities could achieve perhaps as much as 4% greater fuel efficiency if efficiency became more important the cost of power or reliability. This would be achieved by, for example, increasing steam temperatures, decreasing excess air, etc. The right ordinate represents actual efficiencies (based on the higher heating value) of modern, installed, coal-fired systems.